Drilling apparatus is known comprising an extensible mast constituted of a fixed ramp on which is slidably mounted, by means of lateral double-action jacks or cylinders, a movable slide over the length of which the drilling head is guided to drive the tool. Means are provided for synchronization of the raising and lowering movements of the drilling head with those of the movable slide. Such means include a first pair of chains each of which is wound on a wheel or pulley mounted at the upper part of the movable slide and is fixed at one of its extremities to the drilling head and at the other of its extremities to the upper portion of the fixed ramp, and a second pair of chains, each of which is wound on a wheel or pulley mounted at the lower portion of the movable slide and is fixed at one of its extremities to the drilling head and at the other of its extremities to the upper portion of the fixed ramp. Such machines have been developed for drilling bores for mining exploitation and are adapted to the formation of holes of relatively small diameter at small depths of the order of several meters for placement of explosive charges in the holes for producing shock waves in the ground.
Such machines are totally inapplicable for drilling or boring to great depths for which they have not been studied.
In fact, the double-action cylinders connecting the fixed and movable portions of the masts are disposed on each side of the said portions and thereby cannot resist the substantial forces of raising a drill string or the still more considerable forces resulting from a jamming of the string which would produce a buckling of the cylinders to put them out of service. In addition, the cylinders are fixed at a distance from the base of the fixed ramp whereas the flexible feed lines for the drilling head bear on a drum disposed at a distance from the top of the movable slide and at the interior of this latter such that the useful travel of the drilling head is about 0.880 with respect to the length of the fixed ramp which represents a small gain in comparison with the possibilities offered for a conventional non-extensible mast and therefore only permits utilization of elements of reduced length for the constitution of the drill string. In addition, the misalignment of the length of the return chains of the drilling head and of the axis of the latter, on the other hand, and the axes of the cylinders and the pulleys or the wheels on which the chains are wound, on the other hand, as well as the offset of the load and resistance axes prevent the utilization of such machine for the execution of deep drilling. In addition, the accelerated wear of the contact surfaces resulting from the deficient mode of displacement of the movable parts of the machine, the misalignment of the axes of the drive elements and the axes of the movable portions produces a poor guiding of the drilling string resulting in jamming and rapid destruction of the costly connection means of the elements or elongation of the drill string.
The disasterous effects of these misalignments are amplified by the absence of means for regulating the tension of the chains in a possible application of this machine for deep drilling.
Furthermore, the slidable mounting of the lateral profile of the movable slide on the sides of the fixed advance ramp produces substantial friction at these surfaces thus creating strong resistance to the sliding of the slide while accelerating the wear of the surfaces between which can be encrusted abrasive impurities in the course of operation.
Additionally, the above-mentioned machine is provided with flexible feed conduits bearing on a drum mounted in the interior of the movable slide and carried by the axle supporting the pulleys on which the chains are wound at a distance from the extremity of the slide, which has as a consequence, reduction of the path of travel of the drill head while producing suspended loops at the time of retraction movement of the masts, these loops resulting in damage of the conduits and interference with the working operations.